372 THE OUTGO OF ENERGY 



2. Divide the skin over the back in the median 

 line. Eaise the skin on one side until the small 

 nerves which pass across the dorsal lymph sac to 

 innervate the skin come into view. Sever from 

 the surrounding skin a piece about one centi- 

 metre square containing the endings of one of 

 the nerves. Let the isolated piece with its nerve 

 endings remain connected with the body only by 

 the trunk of the nerve. As before, determine 

 the least strength of tetanizing current that will 

 cause a reflex movement when applied to the 

 nerve-endings in the skin and to the nerve-trunk 

 respectively. 



The threshold value for reflex action will again 

 be found lower in the nerve-endings than in the 

 nerve-trunk. 



Summation of Afferent Impulses. Pass two 

 fine copper wires about the frog's foot a centi- 

 metre apart and connect them with the secondary 

 coil. Connect the primary coil through a simple 

 key with a dry cell. Stimulate with regularly 

 repeated make induction currents of such strength 

 that single stimuli cause no reflex contraction. 



Summation of the subminimal stimuli will 

 finally cause reflex contraction. 



Determine that the number of stimuli neces- 

 sary to produce a reflex becomes smaller when (1) 



